The use of metal gate technology is viewed as very desirable for complementary metal oxide semiconductor (CMOS) device technology scaling below the sub 0.1 micron regime. Replacing traditional polycrystalline silicon (“polysilicon”) gate electrodes with metal or metal alloy gate electrodes can significantly eliminate undesired voltage drops associated with polysilicon gate electrodes (e.g., polysilicon depletion effect) and improve device drive current performance. Metal and metal alloy gate electrodes can also reduce the parasitic resistance of the gate line and allow longer gate runners in high performance integrated circuit design for applications such as stacked gates, wordlines, buffer drivers, etc.
Conductive materials have different energies measured conventionally by, their Fermi level. As an example, the Fermi level of a material determines its work function. The intrinsic Fermi level of an undoped semiconductor is at the middle of the bandgap between the conduction and valence band edges. In an N-type doped silicon, the Fermi level is closer to the conduction band than to the valence band (e.g., about 4.15 electron-volts). In a P-type doped silicon, the Fermi level is closer to the valence band than the conduction band (e.g., about 5.2 electron-volts).
Metals or their compounds have been identified that have work functions similar to the work functions of a conventional P-type doped semiconductor substrate. Other metals or their compounds have been identified that have work functions similar to a conventional N-type doped semiconductor substrate. Examples of metals that have a work function similar to P-type doped semiconductor material, include but are not limited to, nickel (Ni), ruthenium oxide (RuO), molybdenum nitride (MoN), and tantalum nitride (TaN). Examples of metals that have a work function similar to N-type doped semiconductor material, include but are not limited to, ruthenium (Ru), zirconium (Zr), niobium (Nb), tantalum (Ta), molybdenum silicide (MoSi), and tantalum silicide (TaSi).